Gallas believes fatigue played a part on Sunday as eight of the team started in the midweek Europa League victory over Panathinaikos.
"Was tiredness an issue? Yes," the Frenchman told the Evening Standard.
"We played on Thursday. Even if we had three days to recover it is not easy, but when you want to win some competitions you have to play every game.
"Only big players can play all the games. We know that so when we go on the pitch we don't think we played three days ago, we just go on the pitch and try to fight.
"However, sometimes it is not easy, especially at this time of year because we play so many games but that is not an excuse.
"What we have to know is that the game is never finished until the referee blows his whistle.
"Yesterday showed that we have to be much more focused at the end of the game.
"We have to accept the defeat yesterday and we have to carry on."